This application claims the priority of German application 196 37 999.7, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for operating an engine brake and to a device for implementing the method. More particularly, the present invention relates to an engine brake operation method for a supercharged internal combustion engine for driving a motor vehicle, in which working gas that is compressed during engine braking, is drawn off prior to the end of the expansion stroke at least partially from the working chambers of the engine through at least one throttle valve, with an exhaust turbocharger of the engine having an exhaust turbine with variable turbine geometry.
DE 39 35 367 A1 describes a method for operating an engine brake for a supercharged internal combustion engine for driving a motor vehicle. Working gas compressed in engine braking operation, prior to termination of the expansion stroke, is extracted at least partially from the working chambers of the internal combustion engine through a throttle valve, with an exhaust line being cut off from the atmosphere and with all of the working gas leaving the working chambers in all operating strokes being fed back in a closed circuit with correspondingly increased pressure to be drawn in once more. In addition, the exhaust turbocharger of the engine has a variable turbine geometry to achieve a boost pressure during engine braking operation that is as high as possible. Additional background subject matter can be found in DE 44 25 956 A1; U.S. Pat. No. 5,091,857; U.S. Pat. No. 5,193,657; and JP 1-60727 (A).
An object of the present invention is to provide a method for operating an engine brake and a device for implementing the method such that engine braking can be achieved with a wider range of application as well as a significant reduction in the wear on the mechanical wheel brakes.
This object has been achieved according to the present invention by a device and method which regulates engine brake power by a regulating device. The device regulates a travel of the throttle valve and/or an intake cross section of the exhaust turbine as a function of operating parameters of the internal combustion engine and of the speed of the vehicle, and the required engine braking power is applied at least preferably through the engine brake by way of the regulator device when there is a signal indicating a braking power requirement by the driver.
One advantage of the method and device according to the invention arises from a predetermined distribution within a certain range or bandwidth of the total braking power between the engine braking power and the mechanical wheel braking power as a result of the throttle valves (exhaust valves and/or decompression valves) regulated variably in terms of valve cross section as a function of the operating parameters of the engine at a corresponding engine rpm.
Another advantage of the present invention arises from a reduction of the influence of temperature on the exhaust system, with the driver employing a corresponding braking power demand to use the variable engine brake.
Still another advantage of the present invention is the ability to time the opening of the valves for the top dead center position of the compression cycle to achieve an increase in engine braking power.
In addition, by virtue of the variably adjustable throttle valve cross section, the volume of air in the engine cylinder which goes through the expansion stroke is determined. Because the ratio between the volume of air leaving the cylinder through the exhaust valves prior to the top dead center position at the end of the compression stroke and the volume of air that remains in the cylinder during the next expansion stroke determines the braking power level, the latter is co-determined by the variable throttle valve cross section directly.
As a result of the high boost obtained during engine braking operation, the charger rpm is at a high level so that, immediately after the braking process, a sufficiently large volume of air is available for firing the engine. Thus, a smoking problem does not occur during the acceleration phase even with the large volume injected.
The present invention achieves maximum engine braking performance by storing the values for the throttle valve travel and the values for the exhaust turbine intake cross section as a function of certain operating parameters of the engine such as rpm, turbine intake pressure, boost air pressure, and ambient temperature for example are stored in an electronic characteristic field. Thereby, these values can be read out from the characteristic field as a function of currently measured engine operating parameters to regulate the engine brake.
The method according to one embodiment of the present invention can be used with particular advantage because the driver during normal driving usually sets the braking power requirement by operating the brake pedal for the mechanical wheel brakes. At this point, the sensor which detects the braking power requirement can also evaluate an intensity (operating speed) for the operation of the mechanical wheel brake possibly in order to provide increased engine braking power immediately.
Another advantage of the present invention is an increase in safety because use of the hand brake during driving activates the maximum possible engine braking power possible with the current operating parameters of the engine. In addition, further redundancy for activating the engine brake can be achieved by using an emergency button.
In the event of deviations from the actual value from the set point that are too great, the present invention detects an error, and the driver is notified by a visual display and/or an acoustic signal that there is an error in the engine braking system and that the vehicle should visit a repair garage. The computer in the engine braking system logs this state. These data can only be deleted at the garage or supplemented by records of the repairs performed.
A considerable reduction in the wear on the mechanical wheel brakes is achieved by the present invention in that, largely independently of the frequency and intensity of the operation of the mechanical wheel brakes by the driver, the total braking power is provided for the most part by the engine brake.
Another advantageous aspect of the present invention protects the valves by minimizing exhaust temperature stress. In addition, by closing the throttle valve in phases, intermediate cooling of the exhaust system can be achieved with relatively high engine braking power. Moreover, the stress imposed by the exhaust temperature can be reduced further if the prevailing rpm conditions permit shifting down to a lower speed.